The plant growth substance abscisic acid is involved in a number of physiological responses, such as closure of stomata, seed maturation and dormancy, amelioration of environmental stresses through gene activation, etc. During water stress there is a large increase in the content of abscisic acid. To understand the many roles of abscisic acid in plants, it is essential to know how its concentration is regulated at the tissue and cellular level. This project deals with the biosynthesis and catabolism of abscisic acid. Biochemical as well as genetic approaches are proposed. The research is based on the hypothesis that all abscisic acid in green plants is derived by oxidative cleavage of 9-cis-epoxy-carotenoids and that the cleavage step is stimulated by water stress. Objectives are 1) to identify the cleavage substrate(s) and products; 2) to prepare a cell-free system for the cleavage enzyme; 3) to isolate novel biosynthetic mutants in Arabidopsis; 4) to purify the enzyme that converts abscisic acid to phaseic acid; 5) to select for a mutant in Arabidopsis that is blocked in the conversion of abscisic acid to phaseic acid; and 6) to determine the mechanism by which different osmotica stimulate abscisic acid biosynthesis in the wall-less alga Dunaliella. Once the pathways for abscisic acid synthesis and degradation have been worked out, it will be possible to find out at the biochemical and molecular level how a water deficit stimulates abscisic biosynthesis, and how restoration of turgor results in the conversion of abscisic acid to phaseic acid. These studies are therefore of interest in connection with the response of plants to drought.
